IO(4)         FreeBSD/i386 Kernel Interfaces Manual       IO(4)

NAME
     io -- I/O privilege file

DESCRIPTION
     The special file /dev/io is a controlled security hole that allows a
     process to gain I/O privileges (which are normally reserved for kernel-
     internal code).  Any process that holds a file descriptor on /dev/io open
     will get its IOPL bits in the flag register set, thus allowing it to per-
     form direct I/O operations.  This can be useful in order to write user-
     land programs that handle some hardware directly.   Note that even read-
     only access will grant the full I/O privileges.

     In addition to any file access permissions on /dev/io, the kernel
     enforces that only the super-user may open this device.


FILES
     /dev/io

SEE ALSO
     i386_get_ioperm(2),
     i386_set_ioperm(2),
     mem(4)

HISTORY
     The io file appeared in FreeBSD 1.0.

FreeBSD 4.10         January 1, 1996        FreeBSD 4.10

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